It is known to connect devices via a cable to transmit power between them, the device that provides power acting as a power source and the device that receives power acting as a power sink. In this context, a “cable” can comprise any solid physical conducting conduit between devices (a wireless connection that transmits data or power through atmospheric space is outside the definition of a “cable”). One common example of this is the connection of a mobile device such as a cellular telephone or tablet computer with a charger so that a battery of the mobile device can be charged.
The Universal Serial Bus (USB) standard defines cables, connectors and communications protocols for use in connecting hosts and devices, for example, connecting peripheral devices to a desktop or laptop computer. A USB connection between devices can be used both for the exchange of data and also for the transmission of power, which can be for charging mobile devices. When charging, the USB host will usually function as a power source and the USB device as a power sink although these roles can in some cases be reversed. The mobile device comprises a battery which acts as a power sink and can be charged when connected via the bus to the host. The host may be another device such as a desktop or laptop computer that acts as a power source, or it may comprise a charger plug which can plug into a mains electricity source.
In a cable connection between devices, it is important to determine when the power source is attached or detached from a system, so that charging and operational aspects of the connected device can be adjusted in the desired manner. In particular, it is desirable to accurately and quickly detect when a power source has been detached, to make sure that there are no dangerous or undesirable electrical conditions created at the side of the power sink.
In a cable connection between devices, there will be a channel for transmitting power. The channel may comprise a physical wired connection provided by the bus. A detachment of the source is detected when a voltage associated with this power transmission channel falls below a pre-determined threshold.
This works for many scenarios, but there is a problem when it comes to the use of capacitive power converters when charging.
A capacitive power converter comprises a DC-DC converter that can change an input voltage or current to a different output voltage or current using capacitive elements for the storage of charge, instead of non-capacitive energy storage elements such as inductors or transformers. An example of a capacitive power converter is a charge pump.
In a charging context as discussed above, the energy of the battery can back-boost (or reverse boost) the energy storage elements of a capacitive power converter after a cable is detached. The capacitive power converter is lacking an input power source and so it is using the battery's power from the output to supply the power for the input. The transfer of energy is reversed in this case, coming from the output towards the input. This means that the voltage of the power transmission channel does not fall, or at least does not fall enough to trigger the detection of a detachment. In this case, the battery effectively becomes a power source and starts to be depleted while switching operations of the capacitive power converter continue. The subsequent provision of power to the input side of the capacitive power converter (reverse-boost or back-boost) which may create dangerous conditions in the mobile device or system that hosts the battery.